In the manufacture of semiconductor devices, circuit patterns are printed on a substrate wafer (Si, or GaAs) by the following generalized process. A silicon wafer is oxidized to form a thin SiO.sub.2 surface layer. The oxidized surface is then covered by a photoresist material which polymerizes on exposure to UV radiation or electron beams. After applying the photoresist, the wafer is aligned with a photomask bearing a negative of the circuit pattern to be printed and exposed to UV light or an electron beam. Areas where the radiation passes through the photomask are polymerized, and areas where radiation is blocked by the photomask are not polymerized. Unpolymerized photoresist is stripped away. The exposed SiO.sub.2 surface is then removed, exposing the underlying silicon which can then be doped with various impurities to fabricate the semiconductor device itself. Finally, the overlying polymerized photoresist is itself stripped away. Thus, the function of the photomask is to define the circuit pattern on the substrate. Production of complex, integrated circuits involves as many as 12 or more sequences of the above photolighographic process.
Three basic types of photomasks are used in the semicoductor industry. They have evolved with the increasing complexity of integrated circuits. The first type may be termed "high-expansion" and generally utilizes soda-lime (window glass) and white crown glass types having coefficients of thermal expansion .ltoreq.100.times.10.sup.-7 .degree.C..sup.-1. Circuit designs are printed on the mask using film emulsions or by a combination iron-chromium coating. Because of the high expansion coefficient, the photomasks are used in contact with the substrate to minimize distortion effects. This contact leads to erosion of the circuit pattern, and the photomasks can only be used for a limited number of exposures, making their use undesirably expensive. Another difficulty with high-expansion photomasks is that the alkali content of the glass reacts chemically with emulsions so as to limit the attainable resolution. Alkali deposited at the photomask surface due to reactions with atmospheric moisture can also lead to pinholes which affect circuit quality and delamination effects which limit the utility of the photomask itself. Consequently, high-expansion photomasks are primarily used to manufacture devices with large circuit geometries (5-10 .mu.m) which are not characteristic of present state-of-the-art integrated circuitry.
The second category is that of low-expansion photomasks. These are usually borosilicate and aluminosilicate glasses having coefficients of thermal expansion .ltoreq.50.times.10.sup.-7 .degree. C..sup.-1. These lower thermal expansion materials permit noncontact exposure of wafers and thus longer mask lifetime and more critical circuit resoution (2-5 .mu.m). Again, alkali content of these glasss is a critical problem because of its influence in the formation of pinhole defects in the photomask. The connection between alkali content and pinhole defects in photomasks has been discussed by Izumitani et al, "Surface Texture Problems of High Precision Glass Substrates for Photomasks", Hoya Optics, Menlo Park, CA.
The third category of photomasks comprises ultra-low-expansion materials, typically fused silica, with coefficients of thermal expansion below 1.times.10.sup.-7 .degree.C..sup.-1. The very low expansion coefficient is useful as it induces minimal distortion in the applied circuit pattern and thus allows higher resolution. Since fused silica is alkali-free, no alkali-related pitting or other defects occur during photomask fabrication. Unfortunately, fused silica cannot be made in conventional melting units used for multicomponent glasses, is more expensive to produce, and is often of inferior optical quality to what is possible in the low-expansion class of materials. At the present time, integrated circuit production primarily utilizes the first two classes of materials; high-expansion photomasks for low density circuity, and low-expansion materials for more critical applications.
Tables 1 and 2 give a summary of composition and properties of the most widely used low-expansion photomask glasses (LE-30, E-6, CGW7740, and PMG-1).
TABLE 1 __________________________________________________________________________ Properties of Existing Commercial Photomask Materials Comparative Summary - Photomask Substrates Schott Manufacturer Duran Hoya OHARA CGW Designation PMG-1 50 LE-30 E-6 7740 __________________________________________________________________________ n.sub.d 1.5574 1.473 1.532 1.467 1.474 V.sub.d 58.4 65 Density (g/cm.sup.3) 2.87 2.23 2.58 2.18 2.23 Tg (.degree.C.) 641 530 690 540 530 .sup..alpha. 20-100 (.times. 10.sup.-7 .degree. C..sup.-1) 42.4 34 .sup..alpha. 100-300 (.times. 10.sup.-7 .degree. C..sup.-1) 38 25 .sup..alpha. 20-300 (.times. 10.sup.-7 .degree. C..sup.-1) 46 32.5 37 32.5 T.sub.10.sup.7.6 (.degree.C.) 808 815 921 821 Hydrolytic Stability (DIN 12111) Class 1 1 wt. loss (mg/dm.sup.2) 26* Acid Stability (DIN 12116) Class &gt;&gt;4 1 Wt. loss (mg/cm.sup.2) 1366 77* Alkaline stability (DIN 52322) Class 3 2 Wt. loss (mg/dm.sup.2) 244 321* % T.sub.350 nm (5 mm) 71% 78.5% 84% 85% Knoop Hardness 539 657 520 418 Young's Modulus (Gpa) 74.4 63 74 57.5 62.8 Poisson's ratio ? .20 .159 .195 .20 Stress optical coeff. (10.sup.-6 mm.sup.2 /N) 3.45 2.86 Specific Heat (J/g .multidot. K) ? .84 .17 .17 Thermal Conductivity (W/m .multidot. K) ? 1.16 ? 0.96 1.26 Log.sub.10 Vd. resistivity 15 Dielectric Constant 4.7 4.0 5.1 __________________________________________________________________________ *H.sub.2 O = 99.degree. C./1 hr., Acid, .1 NHNO.sub.3, 99.degree. C./1 hr., Alkali = .1% NaOH, 50 atm (270.degree. C.)/5 hr. (J1588211) .sup..alpha. 50-100.degree. C.
TABLE 2 ______________________________________ Composition of Prior-Art Commercial Photomask Glasses Manufacturer Designation Schott Schott Hoya OHARA CGW Wt. % PMG-1 Duran 50 LE-30 E-6 7740 ______________________________________ SiO.sub.2 46.01 80.5 59.28 79.42 80.5 B.sub.2 O.sub.3 11.31 12.8 4.26 15.77 13.0 Al.sub.2 O.sub.3 11.03 2.3 15.41 1.08 2.3 Na.sub.2 O 0.163 3.6 1.29 2.59 4.0 K.sub.2 O 0.31 0.6 0.80 &lt;0.02 MgO 9.21 CaO 4.98 1.33 &lt;1 BaO 13.82 0.96 &lt;1 ZnO 12.04 5.95 &lt;1 PbO 0.92 Sb.sub.2 O.sub.3 0.43 As.sub.2 O.sub.3 0.36 0.9 ______________________________________
As can be seen, all of these glasses have thermal expansion coefficients between 30 and 50.times.10.sup.-7. All also contain alkali, which as mentioned above, is undesirable in view of its deleterious effects on production yields and performance of the final photomask.